Although conventionally, PZT (PbZrO3—PbTiO3 solid solution), which is a ceramic material, has been widely used as a piezoelectric material, PZT contains lead. Thus polymeric piezoelectric materials, which are more environmentally friendly and have improved flexibility, are increasingly being used.
Currently known polymeric piezoelectric materials are poled polymers such as nylon 11, polyvinyl fluorides, polyvinyl chlorides, polyureas, polyvinylidene fluorides (β type) (PVDF), and polyvinyl fluoride-trifluoro ethylene copolymers (P(VDF-TrFE)) (75/25).
In addition to the polymeric piezoelectric materials, interest has focused on use of an optically-active polymer such as a polypeptide or a polylactic acid in recent years. Polylactic acid-type polymers are known to exhibit piezoelectricity only upon mechanical stretching.
Among the optically-active polymers, crystals of polymers such as polylactic acids have piezoelectricity arising from permanent dipoles due to the C═O bonds present in the direction of the helical axis. Especially, polylactic acids have a low volume ratio of the side chains to the main chain and a large number of permanent dipoles per unit volume, and thus can be ideal polymers among the helical chiral polymers.
Polylactic acids, which exhibit piezoelectricity only upon stretching, are known not to need to be poled and to keep the piezoelectric modulus unchanged over several years.
With regard to polymeric piezoelectric materials that include a polylactic acid, it is reported that a polymeric piezoelectric material having a high piezoelectric constant d14 and a high transparency is provided by inclusion of a polylactic acid (see, for example, Japanese Patent No. 4934235 and WO 2010/104196).
With regard to devices that include a polylactic acid-type polymer as a polymeric piezoelectric material, a pressure-sensing touch panel that includes a piezoelectric sheet made from a polylactic acid, for example, is proposed (see, for example, WO 2010/143528, WO 2011/125408, WO 2012/049969, and WO 2011/138903). Pressure-sensing touch panels can sense not only two dimensional positional information on the touch panels, but also pressure applied to the touch panels, and thus the touch panels can be configured for three-dimensional operation.